In such hydraulic lift apparatuses, the pressure fluid source generally is comprised of a constant pump which is driven by an electric motor. The speed of the motor is controlled in response to the positions of a valve lever. This allows to change the raising speed without any substantial throttling losses during the raising of a load. It has become known to make the lowering speed also dependent on the positions of the valve lever; this is achieved by a way valve in the lowering branch. The potential energy of the load is converted into heat energy and the throttle of the way valve and is fed into the tank along with the hydraulic fluid. Furthermore it has become known to use the motor pump unit for performing the load holding function during the lowering operation and to refeed a portion of the potential energy of the load into the battery via the electric motor which operates as a generator in this situation.
From German 20 14 605, German 26 18 046, German 30 18 156, it has become known to set the lowering speed by means of hydraulic reservoirs or by means of throttles. As an alternative, the motor operates only in one operative mode. As a result, the electric motor has only a very limited operative range when it operates as a generator. From U.S. Pat. No. 3,947,744, it has become known to control the electric motor by a simple field control during the lowering operation, and it has become known from German 36 02 551 to use a series-wound motor, which operates in a limited operative range. The operative ranges which are not covered by the electric motor driven as a generator have to be covered by further hydraulic throttles; it is inherent that the potential energy of the load cannot be used in this case.
In lift trucks it is generally necessary to feed hydraulic pressure to additional consumers. It has become known to pressurize these hydraulic consumers also via the hydraulic pump by setting a constant speed value for performing the secondary functions, with a valve lever position responsive speed component being added to the constant speed value for performing the raising function. During the lowering operation, a hydraulic pump cannot operate as a generator and perform the secondary functions at the same time. Accordingly, it has become known from U.S. Pat. No. 3,947,744 to provide an additional motor pump unit for performing the secondary functions.
It is an object of the present invention to provide a hydraulic lift apparatus for a battery driven lift truck wherein the drive machine for the hydraulic pump is driven over the complete operative range required by the hydraulic system for raising and lowering the load, while no additional motor pump unit is required for performing the secondary functions.
The present invention and further developments of the invention are defined in the patent claims.
According to one aspect of the invention as defined in patent claim 1, an externally excited direct current machine is provided which, according to one embodiment of the invention, provides for controlling of the excitation and armature voltages independently of each other. To this end, there is provided a separate field current regulating means including a desired value generator which determines a desired value of the field current from predetermined relationships of the speed and the armature current power switches actuatable by said regulating means are associated with the field coil and the armature, the arrangement and operation of said power switches determining the amount and the direction of the current through the armature and the field coil, and control means comprises a directional means for the raising and lowering operations. Furthermore, it is of inventive importance that the lowering branch is provided between the valve assembly and a connection between a check valve and the inlet of the hydraulic pump, with the hydraulic pump pressurizing the secondary hydraulic consumer. As a result, the direct current machine is driven at all times in the same direction of rotation, no matter whether there is a raising or lowering operation. Accordingly, the hydraulic consumer can be operated directly by the hydraulic energy from the lowering operation, so that efficiency losses due to additional energy conversions are avoided.
The present invention allows for a load independent control of the lowering speed both when the direct current machine operates as a generator and when there is a controlling operation solely via the valve assembly. The load holding function can be realized by the manually actuatable control valve which is infinitely adjustable and accordingly allows for a very sensitive regulation of the raising and lowering speeds.
During the lowering operation, the normal case is that the secondary hydraulic consumer is operated by the hydraulic energy resulting from the lowering operation. The excess volume flow is refed into the tank. Furthermore, it is possible that, during the lowering operation, the pressure is greater than necessary for operating the secondary consumer, while only a too small volume flow is available. The speed control causes the electric motor to drive the pump.
In order to prevent that the lowering speed will become excessive, one embodiment of the invention provides a hydraulic volume flow limiter in the lowering branch, which limiter limits the volume flow from the hydraulic cylinder at the inlet to a value predetermined by the valve lever position. It may be comprised of a pressure balance which is controlled by the volume flow and the inlet pressure of the control valve assembly. This constellation ensures that the lowering speed is maintained to be substantially constant. As soon as the flow rate in the pressure balance exceeds a predetermined value relative to the valve lever position at the control valve, the pressure balance starts its controlling function. It operates in connection with the throttle of the control valve as a two-way flow regulator and accordingly maintains the said lowering speed at a constant value. In this manner, it is ensured that the hydraulic pump can draw fluid via the check valve. Preferably, in this mode the control is such as to obtain a constant speed of a value such that the volume stream fed by the pump is sufficient to accommodate the maximum volume flow demand of the additional consumer. A valve lever position responsive velocity or speed of the secondary consumer is adjusted via an additional control valve, with the excess volume flow being refed into the tank.
Finally, it is also possible that both the pressure and the volume flow during the lowering operation are smaller than required for the secondary function. In this case, an embodiment of the invention provides that a switch valve is connected between the pressure limiter and the hydraulic pump. The switch valve directs the hydraulic fluid directly to the tank when the pressure and volume flow in the lowering branch are smaller than necessary for operating the hydraulic consumer. The combination of the pressure balance and switch valve then works as a two-way flow rate regulator. The pressurizing of the secondary hydraulic consumer occurs in the same manner as already described.
The lift mast of a hydraulic lift truck includes among others a free lift cylinder and at least one mast lift cylinder. The oil volume of the free lift cylinder is discharged during the lowering operation only after the mast lift cylinder has been completely retracted and accordingly does not contain any oil. As a result thereof, there will be a transition between the lowering operation in the mast lift mode and in the free lift mode. Due to the different cylinder faces in the mast lift mode and free lift mode there will be different lowering speeds at the same generator speed. In order to compensate for this, a further embodiment of the invention provides for detecting whether the lift mast is in the mast lift range or in the free lift range, and in response thereto proportionality factors between the desired lowering speed value and the motor speed in the control are changed so as to obtain the same lowering speed both in the mast and free lift modes.
From "Microprocessor-based High-Efficiency Drive of a DC Motor" in IEEE Transactions on Industrial Electronics, vol. IE 34, No. 4 November 1987, pages 433 to 440, it has become known to control or regulate the armature and the field of a direct current machine so that the desired value for the field current is determined by predetermined relationships between the speed and the armature current, i.e. the actual armature current value. In this connection, a corresponding algorithm or a corresponding table is to be provided.
The control or the desired value generation for the lift apparatus is obtained by an electric signal, for example via a manually actuated potentiometer, with directional means providing signals indicating the operations raising or lowering, respectively. At the moment when the hydraulic fluid starts to flow in the lowering branch via the control valve assembly, the hydraulic pump drives the direct current machine which operates as a generator. Since, however, the desired value for the speed still equals zero, the control tries to obtain this value, whereby the associated power switch for the armature is completely fired. The power switches for the field coil are operated such that the current is at a maximum. From this results a maximum brake torque which is sufficient to lower the load at a minimum speed if this is desirable. By providing corresponding desired values for the speed on the other hand, the raising and lowering speeds can be adjusted to the desired value.